"Jelena Ostapenko has perfected the 'no look' handshake," she tweeted.

Asked in the post-match press conference if her and Kasaktina were friends, Ostapenko had this to say: "I actually don't know her. We just play the same tournaments from the age of 10 or 12. I saw her many times, but I don't know her."

The Latvian 12th seed, who blasted her way to the Roland Garros title in June, got off to a fast start but the unseeded Kasaktina mixed her game with sliced backhands and chopped forehands to slow her opponent down.

"In the beginning I was a little bit nervous but I tried to stay focused and play my game and it paid off," said Kasaktina, who faces Estonian qualifier Kaia Kanepi for a place in the quarter-finals.

World No.1 Karolina Pliskova, mewanwhile, fought off a match point to claim a riveting 3-6 7-5 6-4 comeback win over Zhang Shuai. She faces American Jennifer Brady early on Monday morning.

The runner-up at Flushing Meadows last year is chasing her first grand slam title.

Men's world No.1 Rafael Nadal recovered from being a set down for the second match in a row to reach the last 16 for a ninth time, beating Argentine Leonardo Mayer 6-7 (3-7) 6-3 6-1 6-4.